The objectives of this study are to: (1) examine in the conscious dog the role of the baroreflex in the maintenance of arterial pressure during acute and chronic periods of ventricular bradycardia;; (2) to assess the acute and chronic effect of low ventricular rate on the baroreflex control of heart rate and peripheral resistance; (3) to determine and compare the time course of resetting in the baroreflex control of heart rate and the reflex control of perifpheral resistance. The complete heart block dog will be used in this study because this preparation affords the means to acutely or chronically change the output from the heart by changing the pacing rate. Using this model, reflex control of the circulation can be examined inthe conscious animal by varying the rate of the pacemaker. There is little know about the effect of heart block on baroreflex function or the role of the baroreflex in the regulation of arterial pressure in this disease. The function of the carotid sinus and of the aortic arch in the regulation of the circulation in heart block will be determined by selective denervation of each area and measuring reflex sensitivity. Twenty four hour recordings of ECG, atrial ECG and arterial pressure will be made during the course of these experiments to obtain a complete picture of the effect of ventricular rate, and baroreceptor denervation on the circulation.